A currently-used inkjet printing apparatus is known having one printing unit with a plurality of printing heads arranged in a transportation direction of a recording medium (e.g., web paper). See, for example, Japanese unexamined Patent Publication No. 2014-24266A. Such a printing apparatus described in the above Patent Literature has the printing heads for colors black (Bk), cyan (C), magenta (M), and yellow (Y) that are arranged in order in the transportation direction.
With the printing unit having the above configuration, the printing heads adjacent in the transportation direction have different printing timings, thereby allowing printing with corresponding colors at positions in response to image data. A printing timings is determined in accordance with a transportation velocity of the recording medium and a distance between the adjacent printing heads.
However, the example of the conventional apparatus with such a configuration has the following drawback. That is, in the inkjet printing apparatus, the printing timing is controlled in accordance with a signal detected by an encoder. The encoder is provided in a roller contacting the recording medium. Examples of the recording medium include one subjected to coating printing, or having one face printed and dried prior to the printing by the printing heads. Here, the recording medium is heated due to drying. Transportation of the heated recording medium causes the roller to be heated and thus to be expanded thermally. This leads to a delay of the detection signal from the encoder. Consequently, discharge timings of ink droplets from each of the printing heads are shifted. As a result, a color shift occurs in the printed recording medium.
Description will be made with reference to FIGS. 1A to 1C and FIGS. 2A to 2C. FIGS. 1A to 1C each illustrate a flow chart of printing when a roller with an encoder is not expanded thermally. FIGS. 2A to 2C illustrates a flow chart of printing when the roller is expanded thermally.
In FIGS. 1A to 1C, a roller 81 has a perimeter L, and paper 82 is transported at a velocity V. The perimeter L of the roller 81 and the transportation velocity V have a relationship of V=L/t. Here, it is assumed that inkjet heads 83 for various colors are each arranged by a distance L, and a first inkjet head 83 is disposed downstream by a distance L from the roller 81. That is, a distance between the roller 81 and the inkjet head 83 for black (Bk) is denoted by L, and a distance between the roller 81 and an inkjet head 83 for yellow (Y) is denoted by 4L. The encoder outputs a timing reference signal in synchronization with rotation of the roller 81, whereby ink droplets are discharged from each of the inkjet heads 83 in accordance with the signal.
The following describes printing of ink droplets in different colors onto the paper 82 on the same line in the transportation direction. As illustrated in FIG. 1B, after the paper 82 passes the roller 81, the inkjet head 83 for black receives a discharge signal at time t, thereby printing a dot 84 in black. Moreover, a discharge signal is inputted at time 4t, whereby a dot 85 in yellow is printed. This allows printing of the dot 84 in black and the dot 85 in yellow onto the paper 82 on the same line in the transportation direction, as illustrated in FIG. 1C.
On the other hand, it is assumed as illustrated in FIG. 2A that the roller 81 provided with the encoder is expanded thermally to cause its perimeter to increase to be of L+a. However, the transportation velocity V of the paper 82 is controlled constant to be V=L/t. Consequently, an angular velocity of the roller 81 becomes low, and a time interval of the timing reference signals from the encoder becomes longer. As a result, a shift in discharge timing occurs among the inkjet heads.
Accordingly, as illustrated in FIG. 2B, after the paper 82 passes the roller 81, the inkjet head 83 for black receives a discharge signal at a timing t+t′. Correspondingly, the dot 84 is shifted backward in the transportation direction by a distance where the paper 82 travels during time t′. Moreover, the inkjet head 83 for yellow receives a discharge signal at a timing 4(t+t′). Correspondingly, the dot 85 is shifted backward in the transportation direction by a distance where the paper 82 travels during time 4t′. As a result, as illustrated in FIG. 2C, a positional shift occurs between the dot 84 in black and the dot 85 in yellow although the both dots should be printed on the same line. Here, the time t′ has a relationship of t′=a/L·t. Thus, the dots 84 and 85 are printed while being shifted backward by distances a and 4a, respectively, each from a position to be printed on the image data. Accordingly, a shift 3a occurs between the inkjet head 83 for black closest to the encoder and the inkjet head 83 for yellow furthest from the encoder. This causes an increased color shift.